Dielectric Properties of Nylon 6/NBR...
Transcript of Dielectric Properties of Nylon 6/NBR...
Dielectric Properties ofNylon 6/NBR Composites
Rajesh C. “Development and characterisation of Nylon Fibre Reinforced NBR composites” Thesis. Department of Chemistry, University of Calicut, 2007
Chapter 6
Dielectric Properties of
Nylon 6lNBR Composites
The dielectric properties such as dielectric constant, volume resistivity and dielectric
loss factor of nylon 6Jibre reinforced NBR composites have been studied as a function
of frequency at dlflerent fibre loadings. The eflects of curing systems and bonding
agents on the dielectric properties have also been studied. The dielectric constant
decreases with increase in frequency; attributed to the decreased orientation
polarization at higherfrequencies. The dielectric constant values have been found to be
lower for fibre filled systems than the gum. The DCP cured composite samples exhibit
higher dielectric constant than the corresponding sulphur cured systems. The addition
of bonding agents reduces the dielectric constant of the composites. The volume
resistivity of the composites increases with the addition of fibres and with the
incorporation of the bonding agents. The addedjbres and the dzferent bonding agents
decrease the dielectric dissipation factor of the matrix.
Chapter 6: Dielectric Properties
6.1 INTRODUCTION
Polymers are, in general, good electrical insulators with volume resistivities up to
1020 ohm m. They are commonly used in the electronics industry as housings or
assemblies. Electrical conductivity is an important factor in many rubber and plastic
compounds which are used for antistatic applications, wire and cable sheathing, and
shielding against electromagnetic interference1-3. For some applications polymers
are made conductive by adding conductive materials like metals, carbon black,
fibres and so forth. Incorporation of a conducting polymer into a host polymer
substrate to develop a blend, composite or an interpenetrating network has been
widely used as an approach to combine electrical conductivity with the desirable
physical properties of a polymer4-7. The process of conduction through a polymeric
system depends upon its composition, chemical structure, physical texture,
morphology and the conditions of measurement8.
Dielectric properties such as the dielectric constant and the dielectric loss reveal
significant information about the chemical and physical states of polymers. These
properties are significantly affected by the presence of another polymer or a dopant
in the polymer9-13. Miyauchi and ~ o ~ a s h i l ~ explained the electrical properties and
the conduction mechanism of polymer-filler particles using polymer grafted carbon
black. Burton et al.I5 made the electrical and electromechanical measurements of
carbon black filled NR. Modification of dielectric and mechanical properties of
rubber - ferrite composites containing manganese zinc ferrite has been studied by
Mohammed et all6. Todorova et al.17 investigated the electrical properties of
elastomer composites filled with titanium diboride. The electrical and mechanical
Chapter 6: Dielectric Properties
properties of conductive rubber composites derived from different blends of EPDM
and NBR containing acetylene black were analysed by Sau et all8. The electrical
resistivity of SBR-carbon black composites has been examined by Mohanraj et all9.
Bishai et investigated the electrical conductivity of SBR-polyester short fibre
reinforced with different types of carbon black. George et followed the
electrical properties of pineapple fibre reinforced polyethylene composites.
In many cases the analysis of dielectric properties provides a measure of the
amorphous fraction of the material which is sensitive to orientation effects, mobility
and to the number and interaction of participating dipoles. Dutta et reported the
studies on the mechanical and electrical anisotropy of pineapple fibres. They found
a sharp increase in dielectric constant and a fall of loss factor along the fibre
direction compared to the transverse direction. Prasantha Kumar and l210ma.s~~
studied the dielectric properties of short sisal fibre reinforced SBR composites.
They pointed out that the chemically treated fibrous composite systems exhibited
lower dielectric constant values compared to the untreated ones. The modeling of
the dielectric properties of wood-polymer composites, by considering them as
multi-component layered systems, was done by Hoflhan et a124.
NBR has only moderate insulating properties and higher values of permittivity
(17-14 ~ m - l ) . The considerably high dielectric loss values (1.33-2.00) cause NBR
to lose its insulating properties. The present chapter deals with the investigation of
the dielectric properties of nylon 6 fibre reinforced NBR composites.
Chapter 6: Dielectric Properties
6.2 RESULTS AND DISCUSSION
6.2.1 Dielectric Constant
Dielectric polarization is the polarized condition of a dielectric resulting from an
applied AC or DC field and it can be expressed as the total induced dipole moment
per unit volume of the dielectric. The total polarizability of the dielectric is the sum
of the contributions due to the several types of displacement of charges produced in
the material by an applied field. Generally, the dielectric constant of a composite
material arises due to the polarization of the molecules.The dielectric constant
increases with increase in polarizability. The dielectric constant (E') of a composite
has contributions from interfacial, orientation, atomic and electronic polarizations.
The interfacial polarization occurs in a composite due to the differences in the
conductivities or the polarizations of the matrix and fillers 25. The orientation
polarization is produced, when polymers containing polar groups are placed in an
electric field 26.
The effect of fibre loading on the dielectric constant (E') values of nylon 6 fibre
reinforced NBR composites as a function of logarithm of frequency is shown in
Figure 6.1. It is evident from the figure that the dielectric constant decreases with
increase in fibre loading at a given frequency. The highest E' values are exhibited
by the gum sample (0 phr). This is due to orientation polarization owing to the
presence of permanent dipoles in the NBR matrix (-C=N group). As nylon 6 fibre is
added some of the dipoles are cancelled by the dipoles present in nylon ()C=O) and
thus the dielectric polarization decreases. Therefore the dielectric constant value
decreases with increase in fibre loading. It is also found that for a given fibre
Chapter 6: Dielectric Properties
loading, the E' shows higher values at lower frequencies. This can be accounted by
the fact that the orientation polarization decreases with increase in frequency. The
complete orientation of the molecules is possible only at lower frequencies and the
orientation polarization requires more time to reach the equilibrium static field
value compared to electronic and atomic polarizations. Therefore as frequency
increases the E' reduces due to the lag in orientation polarization.
log [Frequency (Hz)]
Figure 6.1 Effect of fibre loading on dielectric constant as a function of frequency
Figure 6.2 shows the effect of different curing systems on the dielectric constants of
nylon 6 fibre reinforced NBR composites as a function of frequency. Comparison
has been made with samples containing 24 phr loading of fibre cured by sulphur
and DCP (Samples F and M). It can be seen that the values of dielectric constant
are higher for DCP cured sample than for sulphur cured sample at all frequencies.
This can be attributed to the presence of polar -OH groups in the former. These
Chapter 6: Dielectric Properties
-OH groups are produced during the vulcanisation process initiated by dicumyl
peroxide, where the peroxide free radical (R-0) abstract a hydrogen from the
polymer chain to form R-OH.
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
log [Frequency (Hz)]
Figure 6.2 Effect of curing systems on dielectric constant as a function of Frequency
Figure 6.3 shows the effect of different bonding agents on the dielectric constants of
the composites as a function of frequency. It can be seen that the dielectric constant
values are lower for bonding agent added composites at all frequencies. The
dielectric constant value of phthalic anhydride bonded system is slightly lower than
that of hexa-resorcinol bonded composite. The good interfacial adhesion in
presence of bonding agent removes the voids between the fibres and matrix thereby
eliminating the pockets for moisture absorption. Thus, the modified interface
reduces the dielectric constant values. In the case of phthalic anhydride bonded
systems, in addition to this, the interfacial adhesion between nylon fibres and NBR
Chapter 6: Dielectric Properties
through the formation of hydrogen bonding involving the bonding agent, also
reduces the polarization and hence the dielectric constant value is lower than that of
hexa-resorcinol bonded systems. The mechanism of interfacial adhesion in phthalic
5
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
log [Frequency (Hz)]
Figure 6.3 Effect of bonding agent on the dielectric constant as a function of frequency
anhydride bonded composites has been explained earlier in Chapter 4 (Scheme 4.2).
Figure 6.4 shows the effect of hexa-resorcinol bonding agent on the dielectric
constant of sulphur and DCP cured composites as a h c t i o n of frequency. It can be
seen that in the presence of bonding agent sulphur cured sample has higher
dielectric constant values than DCP cured sample. This can be attributed to the
difference in total polarizability in the two systems. In the former, permanent
dipoles due to -OH groups in resorcinol are present, whereas in the latter these -OH
groups may be involved in hydrogen bonding with the R-OH produced by the
abstraction of hydrogen from polymer chain by peroxide free radical.
Chapter 6: Dielectric Properties
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
log [Frequency (Hz)]
Figure 6.4 Effect of hexa-resorcinol bonding agent on the dielectric constant of sulphur and DCP cured composites
6.2.2 Volume resistivity
The study of the volume resistivity of an insulating material is important because
the most desirable property of an insulator is its ability to resist the leakage of
electric current. Figure 6.5 shows the plot of volume resistivity as a function of
frequency at different loadings of fibres. The volume resistivity has been found to
be decreased with frequency and increased with fibre concentrations. The increase
in volume resistivity with fibre loading can be attributed to the decreased dielectric
polarization upon the incorporation of nylon 6 fibre into the polar NBR matrix.
Figure 6.6 shows the dependence of electrical conductivity (a), which is the
reciprocal of resistivity, on fibre loading at varying frequencies (10 and 13 MHz). It
has been found that, as fibre concentration increases the conductivity decreases.
Chapter 6: Dielectric Properties
-18 phr + 24 phr
log [Frequency (Hz)]
Figure 6.5 Effect of fibre loading on the volume resistivity as a function of frequency
--c 13 MHz
Fibre loading (phr)
Figure 6.6 Variation of conductivity with fibre loading at different frequencies
Chapter 6: Dielectric Properties
However, the conductivity has been found to be increased with frequency. The
variation of volume resistivity as a function of frequency in sulphur and DCP cured
composites is shown in Figure 6.7. It can be seen that the volume resistivity of
DCP cured sample is lower than that cured by sulphur. This is due to the higher
polarizability in the former.
log [Frequency (Hz)]
Figure 6.7 Effect of curing system on the volume resistivity as a function of frequency
The variation of volume resistivity, for bonding agent added composites, is given as
a function of frequency in Figure 6.8. The dielectric constant depends on the
resistivity by the equation2':
log R (298 K) = 23 - 2 E' (298 K) ...... (6.1)
The equation shows that the electrical resistance of composites decreases
exponentially with increasing dielectric constants. The incorporation of bonding
Chapter 6: Dielectric Properties
agent enhances the interfacial adhesion between the fibres and the matrix, which in
turn reduces the voids at the interface thereby enhancing the volume resistivity. It
can also be seen from Figure 6.8 that the volume resistivities of composites
containing both hexa-resorcinol and phthalic anhydride bonding agents are higher
than that of the unbonded one at the same fibre loading.
log [Frequency (Hz)]
Figure 6.8 Effect of bonding agent on volume resistivity as a function of frequency
Figure 6.9 shows the effect of hexa-resorcinol bonding agent on the volume
resistivities of sulphur and DCP cured composites as a function of frequency. It can
be seen that the volume resistivity of hexa-resorcinol bonded composite cured by
DCP is higher than that cured by sulphur. The difference in behaviour is attributed
to the difference in the total polarizability of the two systems.
Chapter 6: Dielectric Properties
log [Frequency(Hz)]
Figure 6.9 Effect of hexa-resorcinol bonding agent on the volume resistivities of sulphur and DCP cured composites
6.2.3 Dissipation factor
Dissipation factor or loss tangent is the ratio of the electrical power dissipated in a
material to the total power circulating in the circuit. It is the tangent of the loss
angle, directly analogous to the tan 6 function relevant to the dynamic mechanical
testing, describing the relationship between storage (G') and loss (G") moduli
(tan 6 = GU/G'). The visco-elastic nature of NBR matrix creates similarities in the
material responses to both mechanical and electrical stimuli. Under the dynamic
excitation, the independent and measured variables move out of phase (stress and
strain in mechanical tests, voltage and current in electrical tests). Most of the
polymers exhibit more than one region of dielectric loss. The measurement of
dissipation factor (tan 6) of an insulating material is thus important since the loss
tangent is a measure of the electrical energy which is converted to heat in an
insulator. This heat raises the insulator temperature and accelerates its deterioration.
Chapter 6: Dielectric Properties
The effect of dissipation factor (tan 6) as a hnction of the logarithm of frequency at
different fibre loading is given in Figure 6.10. The dissipation factor has been
found to be increased with frequency and decreased with fibre concentration. A
strong relaxation is observed, with a peak at a frequency of 5 MHz, which is due to
the a-transition. The positions of peaks are identical for both filled and unfilled
systems. The addition of fibres decreases the relaxation magnitude at each
frequency. The two prime factors contributing to the loss factor (E") are the dipole
polarization and ionic cond~ctance~~. Most of the polymers exhibit more than one
region of dielectric loss. The numerical value of dissipation factor is determined by
both polarity and carrier mobilitg6. The polarity determines the nature of relaxation
and the relaxation time determines the value of tan 6 at specific frequency for that
relaxation.
--c 6 phr 12 phr
--F 18 phr + 24 phr
Figure 6.10 Effect of fibre loading on dissipation factor as a function of frequency
Chapter 6: Dielectric Properties
It can be seen from Figure 6.10 that, at any given frequency, the intensity of
dielectric loss peak decreases regularly as a function of fibre loading i.e.; higher for
gum (0 phr) and decreases with fibre loading. The increment in dissipation factor
with frequency for the gum sample is associated with its amorphous phase
relaxation. Since NBR contains polar groups it may also be due to dipolar
relaxation. The decrease in tan 6 value with the incorporation of nylon fibres into
NBR matrix can be attributed to the increase in relaxation time. This can be due to
the dipole-dipole interaction between the nylon 6 fibres and NBR, both of which
containing polar groups.
Figure 6.1 1 shows the effect of different curing systems on the dissipation factor, as
a function of the logarithm of frequency. It can be seen from the figure that the
dissipation factor values of DCP cured sample is higher than that of sulphur cured
sample.
--c Sulphur cured
log [Frequency (Hz)]
Figure 6.11 Effect of curing system on dissipation factor as a function of Frequency
Chapter 6: Dielectric Properties
The effect of bonding agents on the dissipation factor (tan 6) as a function of the
logarithm of frequency is given in Figure 6.12. The dissipation factor of both hexa-
resorcinol and phthalic anhydride bonded composites are lower than that of the
unbonded one.
-
--c Hexa-resorcinol bonded
I I l I
Figure 6.12 Effect of bonding agent on dissipation factor as a function of Frequency
Figure 6.13 shows the effect hexa-resorcinol bonding agent on the dissipation factor
of sulphur and DCP cured composites as a function of frequency. It can be seen that
the dissipation factor of hexa-resorcinol bonded composite cured by sulphur is
higher than that cured by DCP. The difference in the dissipation factor values of
bonded composites can be attributed to the difference in the relaxation magnitudes.
Chapter 6: Dielectric Properties
0.30
0.00 5.0 5.5 6.0 6.5 7.0 7.5
log [Frequency (Hz)]
-
-
-
,' DCP cured -bonded
-+- Sulphur cured-bonded
-
I I I I
Figure 6.13 Effect of hexa-resorcinol bonding agent on the dissipation factor of sulphur and DCP cured composites
6.3 CONCLUSIONS
The dielectric properties such as dielectric constant, volume resistivity and
dielectric loss factor of short nylon 6 fibre reinforced NBR composites have been
studied as a function of frequency at different fibre loadings. The effect of curing
systems and bonding agents on the dielectric properties has also been studied. The
dielectric constant values have been found to be lower for fibre filled systems than
gum. Composite sample cured by DCP exhibited higher dielectric constant value
than that cured by sulphur which can be attributed to the presence of polar -OH
groups in the former. The addition of the bonding agent reduced the dielectric
constant of the composites. It has been found that in the presence of hexa-resorcinol
bonding agent the sulphur cured sample has higher dielectric constant values than
DCP cured sample which has been attributed to the difference in the total
Chapter 6: Dielectric Properties
polarizability of the two systems. The volume resistivity of the composites was
found to be increased with the addition of fibres and with the incorporation of
bonding agents. The conductivity of the composites decreased with increase in fibre
loading. The added fibres and different bonding agents decreased the dielectric
dissipation factor of the matrix. A dielectric relaxation has been observed at a
frequency of 5 MHz.
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